This invention relates to a radar signal processor and more particularly, to an adaptive radar signal processor which makes it possible to detect a target under optimal conditions suitable for a space environment to be searched.
The most important problem for a radar is to detect a target signal with a high detection probability from a variety of clutters such as ground clutter from fixed objects such as ground and mountains, weather clutter from clouds and rain, sea clutter from ocean waves, angel echo from a flock, and so forth.
The MTI (Moving Target Indicator) technique has been used to cope with the ground clutter and the LOG/CFAR (Logarithmic/Constant False Alarm Rate) technique, for other clutters. The MTI technique is based upon the premise that ground clutter is in a steady state. In practice, however, "fluctuation" resulting from radar hardware instability or "fluctuation" resulting from swaying objects contained in the fixed object such as woods exists and the ground clutter is not always in the steady state. In such a case, clutter residue occurs.
On the other hand, the LOG/CFAR technique processes signals on the assumption that the statistical property of the clutter follows the Rayleigh distribution. It is reported that weather clutter follows the Rayleigh distribution, however, the other clutters follow the Log-normal distribution or Weibull distribution which encompasses the Rayleigh distribution as a special case. Hence, clutter residue still occurs in accordance with this technique.
As described above, the conventional radar apparatuses assume that the property of the clutter is in the fixed (steady) state, and carry out signal processing corresponding to that state. However, because the clutter property actually varies, clutter residue occurs and target detection with a high detection probability is difficult.
An adaptive array automatically generating the null of antenna directivity in the arriving direction of interference waves (e.g., SIDNEY P. APPLEBAUM, "Adaptive Arrays", IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. AP-24, No. 5, Sept. 1976, pp. 585-598) and adaptive techniques such as an adaptive MTI technique that changes the cut-off range of a filter to match it with the clutter Doppler frequency have been proposed. However, the former can not detect the target on the clutter. Since the clutter Doppler frequency spread changes variously in accordance with the properties of the clutter, the clutter can not be removed sufficiently by merely changing the cut-off range as in the adaptive MTI technique.